Interleukin-2 (IL-2) is a lymphokine which is produced by normal peripheral blood lymphocytes and is present in the body at low concentrations. It induces the proliferation of antigen or mitogen stimulated T cells after exposure to plant lectins, antigens, or other stimuli. IL-2 was first described by Morgan, D. A., et al., Science (1976) 193:1007-1008 and originally called T cell growth factor because of its ability to induce proliferation of stimulated T lymphocytes. It is a protein with a reported molecular weight in the range of 13,000 to 17,000 (S. Gillis and J. Watson, J. Exp. Med. (1980) 159:1709) and has an isoelectric point in the range of 6-8.5. It is now recognized that in addition to its growth factor properties it modulates various in vitro and in vivo functions of the immune system. IL-2 is one of several lymphocyte-produced messenger-regulatory molecules that mediate cellular interactions and functions.
IL-2 was initially made by cultivating human peripheral blood lymphocytes (PBL) or other IL-2-producing cell lines, see, for example, U.S. Pat. No. 4,401,756, and Mertelsmann, et al. European Patent Publication No. 92,163 and U.S. Ser. No. 603,580, filed Apr. 25, 1984. However, recombinant DNA technology has provided an alternative to PBLs and cell lines for producing IL-2. For example, Taniguchi, T., et al., Nature (1983) 302:305-310 and Devos, R., Nucleic Acids Research (1983) 11:4307-4323 have reported cloning the human IL-2 gene and expressing it in microorganisms. Various muteins of IL-2 have been reported. For example, see U.S. Pat. No. 4,518,584, to Mark, et al. and U.S. Pat. No. 4,752,585, to Koths, et al.
IL-2 produced in E. coli by recombinant DNA technology is not stably soluble in aqueous solutions at physiological pH (6-8). "Stably soluble" means that the IL-2 must be soluble for a prolonged time period in an aqueous medium. Because of this partial insolubility, various processes and additives have been devised to render IL-2 stably soluble. For example, IL-2 has been subjected to extreme pH changes (see European Patent Application 87100067.5) and has been combined with various surfactants or other additives (see U.S. Pat. No. 4,604,377). However, extreme pHs can be disadvantageous because they may induce severe structural unfolding and refolding which can create incorrect protein conformations or can denature the protein. Using certain additives, such as surfactants, can also be undesirable. Chaotropic agents, such as urea and guanidine work to solubilize IL-2, but must be used at very high concentrations which is also undesirable. Furthermore, when some aqueous formulations containing soluble IL-2 are lyophilized, they may not maintain the IL-2 in solution upon reconstitution. Consequently, there is a need for a pharmaceutical composition which can overcome the problems of the prior art. This composition should: maintain IL-2 in solution even after lyophilization and be free from surfactants or other chemicals which may alter the IL-2 conformational behavior. This need has been fulfilled by the invention that is described below.